LPG is a long periodic change of the refractive index formed in the direction of a fiber axis by irradiating the UV laser to the core of the single mode optical fiber after optical fiber is stripped of the outer resin coating. In this LPG, a period very much longer than the wavelength of the transmitted wavelength-multiplexing optical signal is formed, for example, a period of between 100 μm and several 100 μm.
In accordance with the long period characteristics of this LPG, the coupling of the specific set of different propagation modes is made possible by the propagation of the optical signal through this grating, thus enabling the LPG to be used as a mode coupler. Moreover, LPG has the optical transmission characteristics of plural optical transmission loss peaks at different wavelengths corresponding to the fundamental mode to the Nth mode (N is two or more integers).
As shown in FIG. 1-7, this optical transmission characteristics is generated by the coupling of the fundamental mode through the LPG 72 having a period A formed in the core 71 of the single mode optical fiber 70 with the cladding mode through the cladding layer 73.
Therefore, the power of the optical signals of the propagation modes of the optical fiber can be coupled to the cladding modes of the optical signal 74. As a result of this, most of the power coupled with the cladding mode of the optical signal 74 is lost. Then the LPG can also be used as a non-reflection filter having a peak wavelength of an optical transmission loss.
LPG, as a non-reflection filter mentioned above, is used for gain equalizer of an optical amplifier, such as EDFA, as disclosed in a Japanese Patent Publication No. 2001-124941, for example.
The erbium doped optical fiber (EDF) used in the EDFA is made up of silica doped with germanium oxide (GeO2), which is the refractive index influencing material, in the same way as in a single mode optical fiber and in addition is also doped with erbium oxide (Er2O3:Er3+). A discrete energy level of Er3+ is used for optical amplification. When energy is given to the EDF from the external source, the resultant excited electron is brought back to the ground state by emitting a light energy of different wavelength, and as a result, the incident light is amplified, similar to the Laser mechanism. Since the gain band of the EDFA is comparatively wide, the EDFA can amplify two or more signal light within a wavelength band, and hence is used in the repeaters for the WDM transmission system.
The fiber grating component comprises a single mode optical fiber (SMF) and a fiber grating, which is a periodic change of the refractive index formed in the direction of the fiber axis of this SMF by irradiation of UV laser light to the core layer.
A fiber grating wherein the periodic change of the refractive index is of the order same as the wavelength of the optical signal transmitting through the grating region, for example, of the order of 1 μm, is called short period grating or fiber Bragg grating (FBG).
On the other hand, a fiber grating with a periodic change of within 100 μm to several 100 μm is called a long-period grating (LPG). LPG can couple the propagation mode of the optical fiber with the cladding mode. Due to such characteristics, the LPG components are used in several kinds of optical communication systems, for example, in WDM (Wavelength Divisional Multiplexing) systems as a filter device to control or remove the Amplified Spontaneous Emission (ASE) of the optical amplifier such as EDFA or to compensate the gain-wavelength dependency.
In conventional LPG72, the surface of the cladding layer 73, which is stripped off the coating, is exposed to outside environment as shown in FIG. 1-7. Therefore, the outside environment influences the optical signal 74 of a cladding mode, which propagates through the cladding layer 73.
For instance, the wavelength characteristics of the cladding mode under various outside environments having different refractive indices are shown in FIG. 1-8.
As shown in FIG. 1-8, in LPG 52 designed to have the peak wavelength of optical transmission loss of about 1580 nm, for instance, the wavelength dependence characteristics of optical transmission loss differ sharply in accordance with the difference of the refractive index of outside environment (refractive index n=1, n=1.47, n=1.50). As a result, it was difficult to obtain the desired filtering characteristics.
As shown in FIG. 1-8, the wavelength dependence characteristics of optical transmission loss can be maintained when the refractive index of outside environment is n=1, that is, the outside environment of LPG 52 grating region is air.
Thus, in the conventional LPG, the grating region is uncovered, surrounded by air, and the LPG is adhered in the groove of the glass package.
The grating region protected by such a glass package is further protected by a protection part such as SUS pipe, etc., (second package) and is made as a LPG component.
FIG. 1-9 shows a protection structure of the conventional glass package for the grating region.
As shown in FIG. 1-9, the glass package 60 has the groove 61 of cylindrical form. The optical fiber 63 with the grating region is arranged in the groove 61 of the glass package 60 and is surrounded by air 62. In addition, both the ends of the glass package 60 and of the optical fiber 63 are fixed with adhesive 64.
In making a conventional LPG component, the glass package for protecting the grating region and the adhesive that connects the glass package with the optical fiber are always needed. This raises the cost of the final LPG component.
Moreover, the conventional LPG has a possibility that the wavelength dependence characteristics of the transmission loss might change with the degradation of the adhesives with the passage of time.
Furthermore, in manufacturing the LPG components, the protection process mentioned above by which the grating region is protected by the glass package, the adhesion process using adhesives, an adhesive annealing process and the secondary package processes are needed. Therefore, the number of manufacturing processes increased and the manufacturing cost rose.
In addition, as shown in FIG. 2-8, the center wavelength of LPG component which uses SMF has a big temperature dependency (about 50 pm/° C.) compared with an usual gain equalizer. Since, methods to compensate the above-mentioned temperature dependency of the center wavelength is additionally required when LPG component that used SMF is employed, the use of LPG component as gain equalizer is rare.
Furthermore, in order to use LPG for various optical fiber communications systems, such as a WDM system, the variation of the center wavelength has to be suppressed to about 0.5 nm or less, and the variation of transmission loss has to be controlled to about 1 dB or less, for a design permissible value, for example, from a viewpoint of securing the long-term reliability.
As a result of performing the long-term reliability examination to LPG using SMF based on GR-1221 of Bellcore(Telcordia), as shown in FIG. 2-9, it turns out that center wavelength shifts to about 3 nm to the short wavelength side, and the transmission loss profile is changed.
Therefore, it is needed to increase the long-term reliability of LPG component, i.e., the shift of center wavelength has to be suppressed sharply and the transmission loss profile has to be matched after the long-term reliability examination based GR-1221 of Bellcore(Telcordia).
Moreover, recently, Dynamic Gain Equalizer (D-GEQ) has been developed as a gain equalizer to compensate the variation of gain profile of EDFA with the temperature change or passage of time (dynamic waveform change). This D-GEQ has the transmission loss wavelength characteristics which shows dynamic temperature dependency making it possible to compensate said dynamic change in the profile.
That is, since the temperature coefficient of LPG is the product of the difference of the temperature dependency of the effective refractive index of the core layer and the cladding layer and the period of grating, it becomes large compared with usual gain equalizer, such as the dielectric multi layer filter.
Therefore, the temperature dependency of the transmission loss of LPG proportional to the above-mentioned temperature coefficient also changes greatly compared with a usual gain equalizer.
The use of LPG as D-GEQ to compensate the variation of the gain profile of EDFA with the temperature change or passage of time is currently being researched. This LPG has the temperature dependency of the transmission loss based on the above-mentioned temperature coefficient.
However, the temperature dependency of the center wavelength of LPG which uses the SMF having a core layer doped with GeO2 is about 0.05 nm/° C. as shown in FIG. 3-6. FIG. 3-5 is the refractive index profile of above-mentioned SMF.
The temperature dependency of the center wavelength of about 0.05 nm/° C. of the above-mentioned LPG is insufficient to compensate for the variation of the gain profile of EDFA with the temperature change or passage of time. Therefore, it is necessary to increase the temperature dependency of the center wavelength of LPG.
Moreover, when the difference of power level of the optical signals, which have different wavelength is large, the transmission distance and the transmission band might be decreased due to deteriorated the optical signal in the WDM transmission system. Therefore, it is demanded that an optical amplifier make the gain characteristic of the transmission band flat.
However, the gain characteristic of the transmission band of EDFA (for instance, 1530 nm-1610 nm) has the wavelength dependency. Moreover, the noise caused by ASE which is included in the incident light occurs.
Therefore, it is important that the gain equalizer compensates for the wavelength dependency of the gain profile. In addition, it is important that the gain equalizer suppresses ASE, too. The EDFA is combined with a filter device having a transmission loss profile capable of compensating the gain profile of EDFA, so that the practice of flattening the gain profile of EDFA is used. In particular, as for this filter device, LPG having a period from 100 μm to 500 μm is used.
However, the following two problems exist when the above-mentioned LPG is applied to EDFA.
As for the first problem, the gain profile of EDFA varies by the change of temperature. The change in the metastable energy level of the erbium ions changes the gain profile of EDFA. Because of this, the change of temperature varies the slope of the gain profile. In the above-mentioned gain equalizing method, compensation of the change in the slope of the gain profile with the variation in temperature is not carried out.
Therefore, the adjustment of the temperature of the entire EDFA is necessary, generally. However, when such total temperature adjustment is done, the EDFA can not be miniaturized and also the consumption of electricity becomes very high.
As for the other problem, when power of an input signal fluctuates, this disturbance causes the fluctuation in the state of population inversion in EDFA and thus the gain profile of the EDFA varies. Therefore, when a transmission loss profile of a filter device, which is connected to the EDFA, is fixed, the filter device cannot compensate for the variation in the gain profile of EDFA. As a result, the flatness of the gain profile of an amplified output signal deteriorates.
A dynamic gain equalizer, which can vary the transmission loss profile of filter device in the opposite direction so that the variation in the gain profile of EDFA is cancelled out, is reported.
For example, a dynamic gain equalizer is formed by coating the surface of LPG with a material having large temperature coefficient of refractive index. When the temperature of this coating material is changed by means of heaters, different transmission loss profile can be obtained. In other words dynamic gain equalizer forms a transmission loss profile that seems to cancel it as against a gain profile of EDFA. As a result, flatness of a gain profile of EDFA is improved.
The above-mentioned technique is to improve the flatness of the gain profile by combining the EDFA with a filter device having a transmission loss profile which is opposite in characteristic to the gain profile of EDFA. Though the above-mentioned dynamic gain equalizer can suppress the variation in the gain profile of EDFA, it can not control the slope of the gain profile.
The final slope of the gain profile of EDFA in combination with the dynamic gain equalizer can not be controlled even though the fluctuation in the absolute value of the gain can be suppressed by the dynamic gain equalizer. Therefore, there is a limitation to the flatness of the gain profile finally obtained by using this dynamic gain equalizer.
Moreover, as mentioned above, when the slope of the gain profile is varied by the temperature change of EDFA, it cannot compensate for the variation in the slope by a conventional method of the gain equalization. Then, it is necessary to maintain the temperature of the entire EDFA, thus making the EDFA larger and increasing the power consumption.